docs: def set_I(self, I=None):
          """ Set the current circulating on the coil (A) """
          C0 = I is None
          C1 = type(I) in [int,float,np.int64,np.float64]
          C2 = type(I) in [list,tuple,np.ndarray]
          msg = "Arg I must be None, a float or an 1D np.ndarray !"
          assert C0 or C1 or C2, msg
          if C1:
              I = np.array([I],dtype=float)
          elif C2:
              I = np.asarray(I,dtype=float).ravel()
          self._dmag['I'] = I
          if C0:
              self._dmag['nI'] = 0
          else:
              self._dmag['nI'] = I.size